1. Field of the Invention
The invention relates to a machine for the manufacture of a fiber material web, in particular a tissue paper or hygienic paper web, having at least one pressing gap (nip) which is formed between a shoe pressing unit and a drying or tissue cylinder, and water absorbent carrier band, a water-impermeable pressing band and the fiber material web guided through the at least one gap. It also relates to a method for the manufacture of a fiber material web, e.g., a tissue or hygienic paper web, in which the fiber material web to be dewatered is passed together with a carrier band through a pressing gap (nip).
2. Discussion of Background Information
A plurality of embodiments of a machine of the initially named kind are described in DE-A-42 24 730. In this document, at least two pressing gaps are in each case provided in all embodiments. The main press, which lies to the rear when viewed in the direction of travel of the web, includes in each case a drying cylinder and an associated pressing element. A suction pressing roller or a shoe pressing roller can be provided as a pressing element of this kind.
Two pressing gaps are again also provided in a machine of the initially named kind which is known from DE-A-196 54 197. The rearwardly lying main press is formed by a shoe pressing unit and a drying cylinder.
Thus, in the known machines two or more pressing gaps are always provided. This is considered to be imperative in order to achieve on the one hand a careful dewatering without a squashing, which is demanded in particular in the manufacture of a tissue paper or hygienic paper web, and on the other hand an improved production performance through an increased dry content after the press. In this a careful dewatering of the web without a squashing is achieved with the complete or partial replacing of the roller gap presses by shoe presses as a result of the corresponding gap lengthening or the increase in the pressing time respectively.
A substantial disadvantage of known embodiments includes the relatively high investment and operating costs in comparison with the improvements achieved. In view of the previously usual limiting of the maximum line force to a value in the region of 90 kN e.g. for Yankee cylinders it was always necessary in the known press designs to provide at least two pressing gaps.
The present invention provides a machine and a method as generally discussed above in which as high a dry content and/or specific volume can be achieved while avoiding the above named disadvantages and while largely retaining the quality features which are demanded of the produced web.
In a dewatering press of a machine which serves for the manufacture of a paper and/or cardboard web and which is described in EP-A2-0 852 273 the dwell time of the web in a prolonged pressing gap amounts to less than 12 ms.
In accordance with the invention, the machine includes at least one pressing gap (nip) which is formed between a shoe pressing unit and the drying or tissue cylinder respectively. The length of this pressing gap when viewed in the direction of travel of the web is less than or equal to a value of approximately 60 mm and the resulting pressure profile over the pressing gap length has a maximum pressing pressure which is greater than or equal to a value of approximately 3.3 MPa.
A completely unexpected significant increase both in the dry content and also in the specific volume of the fiber material web is achieved with a development of this kind. This is all the more surprising as the proposed solution departs from the course which was previously taken in the press development, the goal of which was, with the replacement of the roller gap presses by shoe pressing with a prolonged pressing gap, in effecting a careful, slow dewatering as a prerequisite for ideal results. As a result of the development in accordance with the invention many tissue paper machines can now be realized with only one single shoe pressing gap.
In a preferred practical embodiment of the machine in accordance with the invention the pressing gap length is less than or equal to a value of approximately 50 mm and the maximum pressing pressure is greater than or equal to a value of approximately 4.3 MPa. The pressing gap length is defined as he contacted length between the drying cylinder and the associated pressing element and the new carrier band which is passing through.
In this the specific volume is likewise slightly increased. An increase of this kind can for example amount to approximately 5%. A considerable increase is arrived at with respect to the dry content, which holds in particular in comparison with a pressing gap which is formed by a suction pressing roller or by a shoe press with a gap length of for example 120 mm. In this case for example an increase in the dry content by approximately 2.5 to 3% is possible if a constant line force of approximately 90 kN/m is assumed. A value of this kind must not be exceeded in the previously usual Yankee cylinders in view of a maximum permissible mechanical stressing. The invention can thus be used with particular advantage in particular in conversions of existing presses. Thus dry content increases and constant or even higher values of the specific volume (bulk) can be achieved in comparison with the conventional presses in spite of a respective line force limiting.
In a further expedient embodiment the pressing gap length lies in a region of approximately 37 mm and the maximum pressing pressure in a region of approximately 4.8 MPa.
With short shoes it turned out that the angle between the tangent which is applied at the end of the pressing gap to the drying or tissue cylinder respectively and the carrier band which emerges from the pressing gap has a significant influence on the dry content of the tissue web. An advantageous embodiment is distinguished in that in particular for the manufacture of a tissue web this angle between the tangent which is applied at the end of the pressing gap to the drying or tissue cylinder respectively and the carrier band which emerges from the pressing gap is xe2x89xa710xc2x0, in particular is xe2x89xa718xc2x0 and is preferably xe2x89xa720xc2x0. With this there results a dry content increase of approximately 1 to 3% in comparison with applications with suction pressing rollers. In this the pressing band is preferably grooved and/or blind bored.
In accordance with an alternative embodiment of the machine in accordance with the invention, at least one pressing gap is formed between a shoe pressing unit and the drying or tissue cylinder respectively. The length of the at least one pressing gap when viewed in the direction of travel of the web is greater than a value of approximately 80 mm and preferably less than 200 mm, in particular at most 150 mm. The pressure profile which results over the pressing gap length has a maximum pressing pressure which is less than or equal to a value of approximately 2 MPa. In this account is also taken in particular of the fact that in longer shoes the dry content which can be achieved, contrary to expectation, decreases.
In particular in this case it is advantageous when the dwell time of the fiber material web in the pressing gap is greater than or equal to a value of for example approximately 3.5 ms and is in particular greater than or equal to 4 ms. In this the dwell time can in particular be defined by the ratio of the gap or shoe length respectively to the web speed.
The maximum line force which is produced by the pressing gap can lie for example in a range from approximately 90 to approximately 120 kN/m.
In an advantageous practical embodiment of the machine in accordance with the invention the shoe pressing unit includes a pressing shoe which can be pressed against the drying cylinder via a plurality of pressing elements which are arranged adjacently to one another and transversely (cross-wise) to the direction of travel of the web. With this a respective desired pressing force profile can be set in particular at the web edges for the uniformiza-tion of the web properties.
A crepe cylinder, i.e. a so-called Yankee cylinder can in particular be provided as a drying cylinder.
The pressure profile which results over the pressing gap length is preferably asymmetrical.
If the pressing gap length is less than or equal to a value of approximately 60 mm and if the pressure profile has a maximum pressing pressure which is greater than or equal to a value of approximately 3.3 MPa, then the maximum pressing pressure expediently lies in the rear half of the pressing gap length when viewed in the direction of travel of the web.
If on the other hand the pressing gap length is greater than a value of approximately 80 mm and if the pressure profile has a maximum pressing pressure which is less than or equal to a value of approximately 2 MPa, then the maximum pressing pressure can lie in particular in the rear quarter of the pressing gap length when viewed in the direction of travel of the web.
In particular in a pressing gap length which is less than or equal to approximately 60 mm, it is advantageous when the average pressure increase gradient in the section of the pressure profile which extends from the gap beginning up to the maximum pressing pressure in a practically new carrier band is greater than or equal to a value of approximately 40 kPa/ mm, in particular is greater than or equal to approximately 60 kPa/mm and is preferably greater than or equal to approximately 120 kPa/ mm.
The average pressure decrease gradient in the end region of the pressure profile in a practically new carrier band is preferably greater than or equal to a value of approximately 300 kPa/ mm, in particular is greater than or equal to approximately 500 kPa/mm and is preferably greater than or equal to approximately 80 kPa/mm. The average pressure decrease gradient in the end region increases with increasing operating time of the felt. Through this, values of more than 1000 to more than 1600 kPa/m are achieved in a pressing gap in accordance with the invention.
In a preferred practical embodiment the water absorbent carrier band lies in the pressing gap between the water-impermeable pressing band and the fiber material web, with the fiber material web making contact with the drying cylinder.
A felt can be provided in particular as a water absorbent carrier band. A felt of this kind can for example have an areal weight which is less than or equal to a value of approximately 1450 g/m2. For example a felt which is structured in a particular manner as a kind of imprinting sieve or felt which is provided with protuberances, i.e. of a so-called xe2x80x9cimprinting fabricxe2x80x9d or xe2x80x9cimprinting feltxe2x80x9d (see for example W098/00604)) or of a xe2x80x9cpatterning fabricxe2x80x9d or xe2x80x9cpatterning feltxe2x80x9d having a coarsely structured surface can be used. The special carrier bands have an advantageous effect on the specific volume of the produced paper web in particular in combination with a pressing gap in accordance with the invention which has a pressing gap length which is greater than or equal to approximately 80 mm.
The water absorbent carrier band can have a different constitution in the thickness direction. Thus the side of the carrier band which faces the fiber material web can for example have a finer structure than its side which faces away from the fiber material web.
In an expedient practical embodiment the pressing band has a surface which is grooved and/or is provided with blind bores, such as is described for example in DE-A-196 54 198.
In principle a further pressing gap can also be formed at the drying cylinder. In this it can be expedient in certain uses when at least two pressing gaps are provided at the drying cylinder which are formed by a respective shoe pressing unit. In principle it is also conceivable to provide an additional pressing gap ahead of the drying cylinder in the direction of travel of the web.
In an expedient practical embodiment of the machine in accordance with the invention the carrier band and the fiber material web are led over at least one suction device ahead of the drying cylinder in the direction of travel of the web, through which then a corresponding pre-dewatering takes place. In this at least one suction device or suction box respectively can expediently be provided which includes a suction roller and/or a suction shoe.
In a preferred practical embodiment of the machine in accordance with the invention a shoe pressing roller is provided as a shoe pressing unit.
It is also advantageous when the shoe pressing unit includes at least one replaceable pressing shoe.
The method, in accordance with the invention in which the fiber material web to be dewatered is passed together with a carrier band through the pressing gap, includes the fiber material web to be dewatered is subjected to a maximum pressing pressure in the pressure plot of at least 3.3 MPa in the pressing gap for a time duration of at most 3 ms. In this the time duration or dwell time respectively of the fiber material web in the pressing gap is defined by the ratio of the gap length to the web speed.
In an alternative variant embodiment of the method in accordance with the invention in which the fiber material web to be dewatered is passed through a pressing gap together with a band, the fiber material web to be dewatered is subjected to a pressing pressure of at most 2 MPa in the pressing gap for a time duration of at least 3.5 ms.
The invention can be used for example for a crescent former, a long sieve tissue paper machine, a double sieve former, a suction breast roller machine etc.
The input material for the manufacture for example of a tissue web can for example consist of refined pulp. Here the pressing gap in accordance with the invention can be particularly advantageously used for increasing the dry content.
A material input of this kind which serves for example for the manufacture of a tissue web can however also include unrefined pulp. In this case the long pressing gap in accordance with the invention can particularly advantageously be used for increasing the specific volume.
The invention can for example also be used for so-called xe2x80x9ccurled fibersxe2x80x9d. The pulp suspension contains in this case a proportion of fibers which were subjected to a special treatment. In this the celluloid fibers, which are substantially straight or curved in a plane respectively, are curved in such a manner that a spatial fiber shape for example in the manner of a helix arises.
The invention can in particular also be used in a tissue paper machine including at least one so-called xe2x80x9cthrough-air-dryingxe2x80x9d process, with the pressing gap in accordance with the invention in particular being used ahead of a corresponding xe2x80x9cthrough-air-dryingxe2x80x9d device. A combination of this kind is advantageous in particular in regard to a high dry content and to a high specific volume. Through the pressing gap in accordance with the invention on the one hand a high dry content is thereby achieved, through which the operating costs of the energy-intensive drying phase are reduced. On the other hand the web is at most slightly condensed, which means that the specific volume of the web is increased or, respectively, remains unimpaired at higher pressing pressures, through which in particular the xe2x80x9cthrough-air-dryingxe2x80x9d process also becomes more efficient and more economical.
The invention can also be used in particular in the manufacture of multiple layer webs using a single headbox or of the manufacture of multiple layer webs using a plurality of headboxes.
The present invention is directed to machine for the manufacture of a fiber material web. The machine includes a shoe pressing unit, and a cylinder comprising one of a drying and tissue cylinder. The shoe pressing unit and the cylinder are arranged to form at least one press nip. A water absorbent carrier band and a water-impermeable pressing band are provided such that the water absorbent carrier band and the water-impermeable pressing band are guided through the at least one press nip, and the fiber material web is adapted to pass through the at least one press nip with the water absorbent carrier band and the water-impermeable pressing band. The at least one press nip has a length in a web travel direction of less than or equal to approximately 60 mm. A pressure profile which results over the press nip length has a maximum pressing pressure which is greater than or equal to approximately 3.3 MPa.
According to a feature of the present invention, the press nip length can be less than or equal to approximately 50 mm and the maximum pressing pressure can be greater than or equal to approximately 4.3 MPa.
In accordance with another feature of the instant invention, the water-impermeable pressing band may be at least one of grooved and blind bored.
According to still another feature of the invention, the fiber material web can include one of a tissue paper and a hygienic paper web.
The maximum pressing pressure can be exerted in a rear half of said press nip length with regard to the web travel direction.
The present invention is directed to a process for the manufacture of a fiber material web, including passing the fiber material web to be dewatered through a press nip together with a carrier band, and subjecting the fiber material web to be dewatered to a pressing pressure of at least 3.3 MPa in the press nip for a time duration of at most 3 ms.
In accordance with a feature of the invention, the fiber material web can include one of a tissue paper and a hygienic paper web.
Further, the fiber material web can include curled fibers.
The present invention is directed to a process for the manufacture of a fiber material web, that includes passing the fiber material web to be dewatered through a press nip together with a band and subjecting the fiber material web to be dewatered to a pressing pressure of at most 2 MPa in the press nip for a time duration of at least 3.5 ms.